Researchers from ESET discovered two new malware threats that use control servers within the Tor anonymity network

Malware writers are increasingly considering the Tor anonymity network as an option for hiding the real location of their command-and-control (C&C) servers, according to researchers from security firm ESET.

The ESET researchers recently came across two botnet-type malware programs that use (C&C) servers operating as Tor "hidden services."

The Tor Hidden Service protocol allows users to set up services -- usually Web servers -- that can only be accessed from within the Tor network through a random-looking hostname that ends in the .onion pseudo domain extension.

This protocol was designed to hide the real Internet Protocol (IP) address of a "hidden service" from its clients as well as hide the clients' IP addresses from the service, making it almost impossible for either party to determine the other's location or identity.

The traffic between a Tor client and a Tor hidden service is encrypted and is randomly routed through a series of computers participating in the network and acting as relays.

Using Tor to host botnet command-and-control (C&C) servers is not a new idea. The strengths and weaknesses of such an approach were discussed in a presentation at the DefCon 18 security conference in 2010.

Practical implementations of this concept have also been seen in the past. In December, researchers from security firm Rapid7 identified the Skynet botnet of 12,000 to 15,000 compromised computers that were receiving commands from an Internet Relay Chat (IRC) server running as a Tor hidden service. The researchers warned at the time that other malware writers were likely to adopt the design.

Two new malware programs discovered by ESET recently suggest that their prediction was right.

"In July ESET researchers detected two different types of TOR-based botnets based on the malware families Win32/Atrax and Win32/Agent.PTA," ESET malware researchers Anton Cherepanov and Aleksandr Matrosov said Wednesday in a blog post. "Both botnets have form-grabbing functionality for possible further fraud operations."

Unlike Skynet, the Atrax and Agent.PTA botnets use Web, not IRC, servers hidden on the Tor network for command and control purposes.

Atrax can download, execute and inject malicious files into browser processes. Its functionality can be extended through plug-ins that are encrypted locally with an AES key generated from the hardware parameters of each infected computer.

Atrax comes with a Tor client component that gets injected into the local browser in order to route the malware's C&C traffic over the Tor network.

The ESET researchers were able to trick the Atrax C&C server into sending two additional plug-ins to a test system infected with the malware. One of them was designed to steal information entered into Web forms and the other was capable of stealing passwords.

The other threat identified in July, called Agent.PTA, is part of a malware family known since 2012, the ESET researchers said. However, the Tor functionality is a new addition to it, they said.

Like Atrax, Agent.PTA has form-grabbing capabilities and its functionality can also be extended through plug-ins. The malware connects to Web control servers operated as Tor hidden services.

"This year we had already detected TOR-based botnets but during the summer we have observed a growth in the numbers of malware families starting to use TOR-based communications," the ESET researchers said. "The TOR-based botnets make it really hard to pursue investigation and C&C location tracking."

However, even if locating the real IP addresses of the C&C servers is difficult when they are only accessible from within the Tor network, analyzing the malware's communication protocols and command and control traffic is still doable, the researchers said.